Sunday, September 7, 2014

Dreadnoughtus and the true contender for the title of heaviest sauropod

Ever since the 1980s, many dinosaurologists familiar with sauropods have become fascinated by the specter of extravagantly colossal sauropod species, fortified by the discoveries of the giant diplodocids Seismosaurus hallorum (now Diplodocus hallorum) and Supersaurus vivianae in the 1970s and 1980s. When James Jensen announced the discovery of Supersaurus and its junior synonym Ultrasauros, the mainstream media outlets hailed Ultrasauros as the largest and heaviest sauropod that ever lived, and based on the holotype (BYU 9044) and the brachiosaurid scapulocoracoid BYU 9462, Jim Jensen put the estimates of the length and weight of Ultrasauros at 80 to 100 feet long (25 to 30 meters) and 75 tons (75,000 kilograms) respectively. On the other hand, the discoverer of Diplodocus hallorum, David Gillette, estimated the maximum length and weight of D. hallorum at 177 feet long (54 meters) and 125 tons (113,000 kilograms) respectively. However, the giant Morrison sauropods were eclipsed by the Patagonian titanosaurs as regards discussion of the heaviest sauropod taxa, and recent publications (Carpenter 2006; Foster 2003) have put Diplodocus hallorum and Supersaurus vivianae in the 33 to 50 ton range (30,000 to 45,000 kilograms), a far cry from the earlier weight estimates.Now, the media has seized upon the discovery of the latest giant titanosaur to be described, Dreadnoughtus schrani (Lacovara et al. 2014), by hailing it as probably the biggest and heaviest sauropod ever to have roamed the earth. This discovery has provided a new window into the question of which sauropod was the heaviest that ever lived, but to compare Dreadnoughtus against other contenders for the title of heaviest sauropod (Argentinosaurus, Futalognkosaurus, Puertasaurus), I have the opportunity to discuss various weight and size estimates for the giant Patagonian titanosaurs with respect to Equation 1 devised by Campione and Evans (2012), which extrapolates the weight of a quadrupedal animal from the minimum circumference of the shaft of its humerus and femur.When using Equation 1 of Campione and Evans (2012) in order to calculate the body mass of Dreadnoughtus, Lacovara et al. (2014) put the body mass estimate of this genus at 65.4 tons (59,291 kilograms), while noting that the body mass of Dreadnoughtus was probably greater taking into account their histological analysis indicating that the holotype (MPM-PV 1156) was not yet fully grown but still massive in terms of body mass. This is well above the weight estimates provided for Giraffatitan brancai by Benson et al. (2014) but less than the body mass estimate of 134.9 tons (122,400 kilograms) provided for the diplodocoid "Amphicoelias" fragillimus by Carpenter (2006), and less than the weight estimated by Mazetta et al. (2004) for the titanosaur "Antarctosaurus" giganteus. (Since the holotype remains of "Amphicoelias" fragillimus [AMNH 5777] have been lost, it is possible that Carpenter's weight estimate for this species is way off by several tons and "A." fragillimus was not as heavy as estimated by Carpenter.) By contrast, the primitive titanosaur Futalognkosaurus and the turiasaur Turiasaurus weighed 42 tons (38,139 kilograms) and 56.1 tons (50,923 kilograms) respectively.

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Regarding the body mass of Argentinosaurus, Paul (1994) put the weight estimate of Argentinosaurus at 88 to 110 tons (80,000 to 100,000 kilograms), but Mazzetta et al. (2004) revised the weight to 80 tons (73,000 kilograms), while Sellers et al. (2013) put the weight estimate at 91 tons (83,000 kilograms). Although it is not implausible that Argentinosaurus was bigger and heavier than Dreadnoughtus judging from the size of the referred femur (MLP-DP 46-VIII-21-3), the known material of Argentinosaurus comprises only a tiny part of the postcranial skeleton (9.2 percent), insufficient to give a reliable estimate of the body mass of Argentinosaurus using the body mass formula devised by Campione and Evans (2012). Likewise, another giant titanosaur from Patagonia, Puertasaurus, may have been heavier and bigger than Dreadnoughtus, with an estimated weight of 88 to 110 tons (80,000 to 100,000 kilograms) based on the huge size of the dorsal vertebra (Novas et al. 2005), but the available material is too meagre to give a more accurate body mass estimate and more complete remains are needed to determine the size of Puertasaurus. In summary, any possible body mass estimates for "Amphicoelias" fragillimus, Argentinosaurus, and Puertasaurus should be treated with caution when taking these sauropods as candidates for the title of heaviest sauropod. Since "Antarctosaurus" giganteus has femora preserved in its type material, it could be a viable contender for the title of heaviest sauropod. Therefore, it may be parsimonious to treat "A." giganteus and Dreadnoughtus as the heaviest sauropods known to science given that the holotype of the former species was not yet fully mature. Update: A new paper by Bates et al. (2015) suggests that the original weight estimate for Dreadnoughtus was an overestimate and that Dreadnoughtus probably weighed 42.1 tons (38,225 kilograms), rather than 65.4 tons as originally claimed by Lacovara et al. (2014).Benson RBJ, Campione NE, Carrano MT, Mannion PD, Sullivan C, et al., 2014. Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage. PLoS Biol 12(5): e1001853 doi:10.1371/journal.pbio.1001853.

Campione, N., and Evans, D., 2012. A universal scaling relationship between body mass and proximal limb bone dimensions in quadrupedal terrestrial tetrapods". BMC Biology: 15. doi:10.1186/1741-7007-10-60.Carpenter, K., 2006. Biggest of the big: a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus. In Foster, John R.; and Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin 36. Albuquerque: New Mexico Museum of Natural History and Science. pp. 131–138.